This invention relates to a plasma source. The invention also relates to a method of generating plasma, and to an apparatus for coating or cleaning substrates. More particularly, this invention relates to a plasma source in which radiofrequency energy is inductively coupled to both a thermionic-field emitter, thereby generating electrons with broad energy distribution for plasma generation and neutralisation, and a discharge process generating a plasma having ions and electrons.
Such a plasma source can be effectively used in the vacuum processing of thin film coatings during electron beam or thermal deposition. The energy imparted by the source to the growing film is capable of modifying the microstructure producing dense, near stoichiometric films that are impervious to temperature and humidity variations. The refractive index achieved is near that of the bulk materials, thus extending the possibilities for multilayer thin film design.
Substrate heating is superfluous with assisted deposition processes. Low temperature coating is a major process advantage offering low-cost fixturing, time/cost and compatibility with low-temperature materials such as plastics.
Plasma sources are also exceptional for in situ substrate cleaning. In particular, argon cleaning provides physical sputter removal of adsorbed water and residual cleaning solvents. Oxygen cleaning can stimulate chemical removal of hydrocarbons through the formation of volatile species.
A primary application for such sources includes precision optical coatings of oxide and fluoride based deposition materials. Examples include anti-reflection coatings for ophthalmic lenses, high tolerance multilayer dielectric optical coatings for telecommunications and high laser damage coatings.
Currently available plasma or ion sources for assisted vacuum deposition processes are have been described in the prior art, such as, for example, U.S. Pat. No. 4,862,032, EP-A-0463230, WO96/30928, FR-2557415 and in S. Pongratz and A. Zoller, J. Vac. Sci. Technol. A 10(4), p 1897, Jul./Aug. 1992.
Certain commercially available plasma sources have a length such that they require a well in the base plate to ensure that positioning within the vacuum chamber does not mask deposition sources due to excessive source height. Consequently such systems require a specialised vacuum chamber for operation and are not readily retrofittable to other vacuum systems.
In plasma deposition, the term neutralisation refers to a state in which there is a balance of ions and electrons. In the absence of neutralisation (which usually involves a surplus of ions) three deleterious effects can occur:
1. Electrons can be drawn to the beam in short-duration arcs that can eject small particles from the arc location. These arcs can cause damage to a sensitive substrate surface and also introduce contamination into the growing film.
2. The occurrence of the arcs as described in 1 also leads to temporal variation in beam-plasma voltage which causes process variation.
3. Space charge effect which spreads the plasma spatial distribution and also introduces edge effects for dielectric substrates mounted in metal holders.
This effect manifests itself as a film thickness variation.
Plasma/ion sources which rely on only thermionic emission have a very narrow electron energy emission characteristic with minimal lower energy electrons as shown in FIG. 1. This problem is overcome in ion sources through use of a separate supply of electrons injected into the plasma to provide neutralization. Current plasma sources rely upon the thermionic electron emission to provide sufficient electrons with necessary energy to provide neutralisation. This method does not provide adequate control over neutralisation and as such effects 1, 2 and 3 above are encountered.
Other ion source systems employing inductively coupled RF energy have been described (see, for example, U.S. Pat. No. 4,104,875). Such systems are susceptible to conductive deposits on the non-conductive window isolating the inductor from the plasma region. Capacitively coupled RF discharge processes have also been used in ion and plasma sources (see, for example, EP-A-0474584).
All of the plasma/ion sources described above have fixed spatial distribution of the ion/plasma flux at the substrate plane, engineered for ion/plasma overlap over the full deposition area or positioned to provide the best overlap with the evaporant fluxes. Such sources compromise achieving the full benefits of assisted deposition as each application requires a specific match of ion/plasma spatial distribution depending upon coating type, required substrate loading over deposition area, deposition material(s), evaporant source flux and film parameter(s) to be optimised via ion/plasma bombardment.
Moreover, sputtering of the thermionic emitter material causes changes in the emitter spatial profile which varies the spatial distribution of emitted electrons with source operating time and hence the plasma distribution.
A general object of the present invention is to provide a radiofrequency energy driven plasma source with enhanced plasma generation, control and neutralisation. Another object of the present invention is to provide a plasma source which avoids the disadvantages and deleterious features of such plasma sources as described above. Broadly we achieve this by using induction to help generate the ions from an electron emitter of a plasma source.
According to a first aspect of the present invention there is provided a plasma source comprising an inlet for a gas which is ionisable to produce a plasma, an electron emitter for producing electrons for ionising the gas, an RF induction coil at least partially surrounding the electron emitter, and an anode.
Preferably, the plasma source further comprises a cylindrical former of electrical insulator material capable of withstanding high temperature, and the emitter is disposed within the former. In an embodiment, the emitter is in the form of multiple cylinders of thermionic-field emitting material arranged on the circumference of a circle lying within the insulating former. We also prefer that plasma source includes a removable base, said base including at least part of the gas inlet. The base may also include apertures through which electrical wires for the emitter, the anode and the induction coil may extend.
The emitter emits thermionic electrons for the generation of the plasma, when held at a negative potential and subjected to heating. In effect, the emitter acts as a cathode.
In a preferred embodiment, at least part of the electron emitter is dome-shaped. More preferably, the emitter is in the form of a cylinder having a domed top.
In an embodiment, the emitter is in the form of a cylinder of varied width and height with a flat top, thereby allowing the spatial distribution of emitted electrons to be changed.
The anode, which is desirably cylindrical, is preferably concentric with the emitter and axially displaced therefrom, generating a potential difference between anode and emitter. The potential difference between anode and ground and axial magnetic fields causes the plasma to be extracted from the source. More preferably, the axial displacement of the anode from the emitter is adjustable. We prefer that a cap is disposed between the anode and cathode, and we also prefer that the cap has an aperture of variable size.
Preferably, the electron emitter is supported by a conductive support column, by means of which the emitter can be held at a negative potential. The emitter is desirably disposed substantially concentrically within the induction coil and the former, the former being disposed within the induction coil. Preferably the induction coil is water cooled.
The induction coil can be operated to perform a number of advantageous functions. In order to generate the plasma it is necessary to heat the emitter, and this can be achieved by means of the induction coil which can be operated to deliver energy to heat the emitter. There are important benefits to induction heating, as compared with direct or radiative heating. In particular, there is no contact with the workpiece, which makes possible a modular plasma source construction. This can accommodate a range of thermionic-field emitter configurations which provide consistency and controllability which is particularly important to ensure constancy in plasma source output over extended periods of time.
We have found that the induction coil is that is can be operated to deliver energy within the former for the generation of broad energy spectrum electrons for effective neutralisation of the plasma. The radiofrequency excitation of the emitter generates electrons via thermionic and field effects, resulting in efficient plasma generation. Both electron generation effects contribute to the broad energy spectrum of electrons, providing effective neutralisation of the plasma. Thus, in an advantageous embodiment of the invention there is provided a control means for the induction coil to control the induction coil to deliver energy within the former for the generation of broad energy spectrum electrons for effective neutralisation of the plasma
Yet another important function of the induction coil which we have found is that it can be operated to produce a time varying axial magnetic field for enhancement of plasma generation and confinement of said plasma to minimise emitter erosion. The induced axial time varying magnetic field can act to locally shield the emitter from ion bombardment and thereby minimize bombardment of the emitter and resulting emitter. This also minimises resulting contamination from emitter sputtered material of the plasma source and resulting plasma. Thus another advantageous embodiment of the invention involves the provision of a control means for the induction coil to control the induction coil to produce a time varying axial magnetic field.
In turn, minimising the erosion of the emitter preserves the emitter spatial profile which ensures constancy of spatial distribution of emitted electrons and hence resulting plasma spatial distribution.
Additionally, sputtered emitter contamination, which is conductive, tends to deposit on the sidewall of the plasma source and thereby reduces inductive coupling. This effect is minimized by the provision of the time varying electromagnetic field.
As mentioned above, the use of the RF induction coil can cause electrons to be generated by thermionic and field effects. The induced skin effect at the emitter surface provides field enhanced emission whereby the current flow within the skin depth induces a strong localized electric field at the surface of the emitter such that electrons are attracted out of the emitter.
This effect increases effective electron emission by two well known mechanisms: firstly lowering the effective work function at the surface and thereby increasing thermionic emission (Schottky emissionxe2x80x94reference C. Herring et al, Rev. Mod. Phys. 21, 185 [1949]) and secondly, emission through the quantum mechanical tunneling effect by which electrons can leak through the surface potential barrier (referred to as strong field emission, see reference R. Fowler et al, Proc. Roy. Soc. A119, 173 [1928]). The combination of these effects to generate electron emission from the emitter is referred to as thermionic-field emission.
The emitter may comprise a high efficiency emitter material such as tungsten, molybdenum (including coatings which reduce work function and/or modify Fermi level) or lanthanum hexaboride (reference J. M. Rafferty, Journal of Applied Physics, Vol 22, Number 3, p299, Mar. 1953); these can be configured to maximize the thermionic-field emission area and minimize inhomogeneous field effects. Moreover, the induced emitter skin depth is typically a few hundred microns, allowing possible use of relatively thin foil material for the emitter and thereby minimizing thermal mass and consequent inertia in achieving the desired temperature.
It is especially preferred that the coating material is diamond, whose electronic properties are such that when it is biased negatively in a vacuum, electrons are ejected from the surface (reference G. T. Mearini et al, Investigation of diamond films for electronic devices, Surf and Interface Anal., vol. 21, 1994, pp. 138 -143). Thus, in accordance with another aspect of the invention there is provided a diamond coated emitter for a plasma source. The former may also be useful as an ion source for use in ion assisted deposition and as an electron source for use in electron beam evaporation. In diamond the work function is small, perhaps negative (referred to as negative electron affinity). In practice this means that thermionic-field emitters based on diamond consume low power and offer high efficiencies such as those utilised in flat panel display applications.
Specific benefits as an emitter are lower operating temperatures (500xc2x0 to 1000xc2x0 C.) and lower electric fields (orders of magnitude less than conventional field emission materialsxe2x80x94typically 10xe2x88x924V/cm) for field emission.
Use of electrically conductive substrates provide a means of heating an applied diamond film via an induction coil and also application of a negative voltage to the emitter material. It is particularly preferred that the diamond is coated on metals which exhibit carbide formation as a localised interfacial layer as a consequence of low mutual solubility with carbon. Such metals include as Mb Ti, Zr, Ta, Hf, W and Mo. The carbide layer acts as a bonding layer which promotes growth of a chemical vapour deposited layer, and aids adhesion by stress relief at the interface.
It is desirable for the plasma source also to comprise a secondary inlet for said gas, said secondary inlet being arranged to inject the gas into a space within the anode.
In a preferred embodiment, the anode is surrounded by an electromagnet capable of producing a time invariant magnetic field. It is especially preferred that the time invariant magnetic field produced by the electromagnet is deconvoluted from the induced time variant magnetic field. This deconvolution can be achieved by proper choice of the field strength of the electromagnet and the induction coil, and by appropriate spacing of the electromagnet and the induction coil. The aim is to ensure that there is no significant interaction between the magnetic fields of the electromagnet and the induction coil.
The deconvolution of the induced magnetic field from the time invariant electromagnetic field allows separate control of source plasma spatial distribution by the induction coil field and the electromagnet field. Spatial control of plasma flux at the substrate plane is provided by the relative positioning of the electromagnet with respect to the induction coil. A greater spatial spread of plasma flux is achieved by increasing the separation of electromagnet and induction coil.
The anode and the electromagnet are conveniently supported by the former. Preferably, the electromagnet is adapted to slide on and off the anode and is adjustable with respect to displacement from the induction coil.
In a preferred embodiment, the electromagnet is in the form of a coil, the number of turns of which is varied along its length to spatially vary the magnetic field and hence the plasma spatial distribution at the substrate plane. It is also preferred that the electromagnet coil is frequency and phase coupled with the induction coil.
In an especially preferred embodiment, the anode and the emitter have separate electrical supplies. More preferably, said separate supplies have a common earth.
In an embodiment, a reactive gas inlet is located at the top of the anode.
According to another aspect of the present invention there is provided a method of generating a plasma comprising: flowing an ionisable gas in contact with an electron emitter, the electron emitter being held at a negative potential; and heating the electron emitter using a RF induction coil to produce electrons from the emitter which ionise the gas to produce a plasma.
Preferably, the induction coil is operated to deliver energy within the former for the generation of broad energy spectrum electrons for effective neutralisation of said plasma.
Preferably, the induction coil is operated to produce a time varying axial magnetic field for enhancement of plasma generation and confinement of said plasma to minimise emitter erosion.
Desirably, an anode and an electromagnet coil are disposed downstream of the emitter, and the method further comprises producing a time invariant magnetic field with the electromagnet coil which is deconvoluted from the time varying magnetic field produced by the induction coil.
Although it is preferred to use the RF induction coil in all aspects of the invention, it is possible omit it in some embodiments; some examples of this are discussed below.
According to another aspect of the present invention there is provided an apparatus for coating or cleaning a substrate, comprising: a vacuum chamber; a substrate carrier adapted to carry a substrate to be cleaned or carried, disposed within said chamber; means for generating material for coating or cleaning the substrate, disposed within said chamber; and a plasma source as described above.
According to another aspect of the present invention there is provided a plasma source comprising an inlet for a gas which is ionisable to produce a plasma, an electron emitter for producing electrons for ionising the gas, means to generate and deliver energy for the generation of broad energy spectrum electrons from the emitter, for effective neutralisation of said plasma, and an anode.
Said generating means preferably comprises a RF induction coil at least partially surrounding the emitter.
The plasma source according to this aspect of the invention may be provided with any combination of the features of the plasma source described above in relation to the first aspect of the invention.
According to another aspect of the present invention there is provided a plasma source comprising an inlet for a gas which is ionisable to produce a plasma, an electron emitter for producing electrons for ionising the gas, means operable to produce a time varying axial magnetic field for enhancement of plasma generation and confinement of said plasma to minimise emitter erosion, and an anode.
Said operable means preferably comprises a RF induction coil at least partially surrounding the emitter. A heating means must be provided to heat the emitter, and in the preferred embodiment this comprises the RF induction coil.
The plasma source according to this aspect of the invention may be provided with any combination of the features of the plasma source described above in relation to the first aspect of the invention.
According to another aspect of the present invention there is provided a method of generating a plasma comprising:
flowing an ionisable gas in contact with an electron emitter, the electron emitter being held at a negative potential; heating the electron emitter to produce a plasma; and delivering energy within the former for the generation of broad energy spectrum electrons for effective neutralisation of said plasma.
According to another aspect of the present invention there is provided a method of generating a plasma comprising: flowing an ionisable gas in contact with an electron emitter, the electron emitter being held at a negative potential; heating the electron emitter to produce a plasma; and producing a time varying axial magnetic field for enhancement of plasma generation and confinement of said plasma to minimise emitter erosion.
According to another aspect of the present invention there is provided a plasma source comprising an inlet for a gas which is ionisable to produce a plasma, an electron emitter for producing electrons for ionising the gas, and an anode, wherein the electron emitter and the anode are provided with separate power supplies which are preferably connected to a common ground.
A heating means must be provided to heat the emitter, and in the preferred embodiment this comprises a RF induction coil.
The plasma source according to this aspect of the invention may be provided with any combination of the features of the plasma source described above in relation to the first aspect of the invention.
The invention also provides a plasma source having a modular construction thereby providing various configurations to tune output plasma current density and spatial distribution for specific process requirements. Thus according to another aspect of the present invention there is provided a plasma source comprising an inlet for a gas which is ionisable to produce a plasma, an electron emitter for producing electrons for ionising the gas, means to heat the electron emitter, an anode, and a magnet, wherein the components are of modular construction thereby facilitating replacement of the components, and adjustment of the relative position of the components.